Thermodynamics of non-ionic surface-active agent + water systems

Abstract

Measurements of water vapour pressure and heat of mixing have been made at 25°C over a range of composition for the octyl- and decylhexaoxyethylene glycol monoether + water systems. The condensed binary phase diagrams show that only the system given by the higher homologue forms a lyotropic mesomorphic phase (middle phase) at 25°C. In both systems, the thermodynamic properties of the solutions are determined by the interaction of the hydrophilic groups with water. The water activity and heat of mixing are smooth functions with respect to composition with no measurable discontinuities at the mesomorphic phase boundaries. The consequences of this conclusion and its relationship to the structure of middle phase are discussed.